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Aug. 23, 2010 -- There is now biologic evidence to back up the belief that vitamin D may protect against autoimmune diseases and certain cancers.

A new genetic analysis lends support to the idea that the vitamin interacts with genes specific for colorectal cancer, multiple sclerosis, type 1 diabetes, and other diseases, says Oxford University genetic researcher Sreeram Ramagopalan.

The study is published in Genome Research.

When Ramagopalan and colleagues analyzed the binding of vitamin D receptors to gene regions previously identified with different diseases, they found evidence of increased binding for multiple sclerosis, Crohn's disease, lupus, rheumatoid arthritis, colorectal cancer, and chronic lymphocytic leukemia (CLL).

"Genes involved in autoimmune disease and cancer were regulated by vitamin D," Ramagopalan tells WebMD. "The next step is understanding how this interaction could lead to disease."

Role of Vitamin D Supplementation

The role of vitamin D supplementation in preventing these diseases is also not well understood.

Exposure to sunlight is an efficient way to raise blood levels of vitamin D hormone, and food sources of the nutrient include oily fish like salmon, fortified milk, and other fortified foods.

But most people would have a hard time getting the vitamin D they need from food, and the increased use of sunscreen has reduced sun exposures.

By one recent estimate, as many as half of adults and children in the U.S. were deficient in the vitamin.

Current recommended daily vitamin D intake is 200 IU (international units) for those up to age 50; 400 IU for people 51 to70; and 600 IU for those over 70. Most experts say that these doses are too low.

Many experts, including Ramagopalan, say 2,000 IU of the vitamin may be optimal for preventing disease.

Blood levels of the vitamin are measured as 25-hydroxyvitamin D. Levels below 20 nanograms per milliliter are generally considered deficient.

Harvard School of Public Health nutrition researcher Edward Giovannucci, MD, says blood 25-hydroxyvitamin D levels of between 30 and 40 nanograms per milliliter may be about right for reducing the risk of autoimmune diseases and certain cancers.

While he says some people can reach these levels without supplementation, many others would need to take 1,000 to 2,000 IU of the vitamin a day.

"Based on what we know, I think it is reasonable to recommend that people maintain blood levels of around 30 nanograms per milliliter," he says.

Unanswered Questions

But vitamin D researcher JoAnn E. Manson, MD, says it is way too soon to recommend taking much larger doses of vitamin D than are recommended.

Manson chairs the preventive medicine department at Brigham and Women's Hospital in Boston and is principle investigator of a large U.S. study on vitamin D.

Still in the recruitment stage, the five-year, 20,000-person study will explore the impact of 2,000 IU of vitamin D on the risks for a wide range of health conditions, including cancer, heart disease, diabetes, hypertension, and depression. The study will also examine the effects of the fatty acid omega-3.

"I think it is important that we not leap ahead of the evidence in recommending high doses of vitamin D," she says. "We will soon have a better understanding of the optimal doses of vitamin D and the optimal blood levels associated with the best balance of benefits and risks. But right now there are many unanswered questions."